The present invention relates to an amplifier, and more particularly to an amplifier which detects the level of an input signal and changes the operating point of an amplification device of an amplifier circuit and the supply voltage in accordance with the detected input signal level.
Compact disks (CDs) from which digital signals can be reproduced have been widely used nowadays as one source of audio signals, in addition to phonograph records and magnetic tapes in which analog signals are used. Compact disks have various advantageous features in their characteristics, including their dynamic range of ninety and several dB or more.
Thus, it becomes necessary for amplifier circuits to cover a broader dynamic range than the range for conventional sources. To this end, there is an amplifier circuit designed to have its supply voltage large enough to handle a maximum input signal without clipping it by setting the operating point of an amplification device at an optimum fixed value for such a supply voltage.
However, since the operating point and the supply voltage of such an amplifier circuit are primarily set to handle such a large input signal level with minimum distortion, a large amount of operating current, which would not be needed otherwise, will flow even with a small input signal level.
In addition, as a matter of fact, the period during which a maximum input signal is driven into the amplifier circuit is relatively short, thus a large amount of idle current will pass through the amplification device of the last stage at most other periods, and this results in unfavorable power consumption, poor amplifier efficiency and durability.
There is an amplifier proposed by the same applicant, which is directed to solve these problems, whereby an analog input signal level applied to the amplifier circuit is detected to control the operating point of an amplification device of the amplifier circuit and control the supply voltage in accordance with the detected input signal level.
With such a proposed amplifier, however, the analog input signal level is detected when it exceeds a predetermined level and in response to the detection, a switch is controlled to supply a higher supply voltage to the amplifier circuit. Therefore, the supply of the higher voltage has an inevitable delay with respect to the excess of the input signal to be detected.
Particularly in the case where the analog input signal is a music signal, the rise in signal level is steep and pulsive, and the time during which a large signal level is applied is very short. Therefore, there arises a problem of causing signal dropouts, distortion and the like.
Further, in order to make a supply voltage rise quickly when it is switched from a lower one to a higher one, it is necessary to have an expensive capacitor of large capacitance, resulting in high cost to the amplifier.
Furthermore, since it is necessary to switch a supply voltage at high speed, semiconductor devices such as a transistor and a diode are used as a high speed switch. However, use of such a semiconductor element and a diode will increase the impedance of the power supply circuit with respect to the operating signal frequencies, thereby deteriorating the sound quality. Moreover, noises are generated from the semiconductor and the diode during switching.
Still further, in determining the operating conditions of the amplifier circuit of a conventional amplifier, it has been solely dependent upon the analog input signal level to be detected. However, in practice, the operating conditions of the amplifier circuit may vary with a manual adjustment of a level adjuster or so-called volume control by a listener even if the analog input signal level is constant. Thus, controlling the operating point of the amplification device of the amplifier circuit and the supply voltage fed to the amplifier circuit based on a detected analog input signal level alone is not adequate and may lead to signal distortion.